Open a web browser like internet explorer or Netscape. Enter the ip address of your router in the address bar of your browser. In the picture above the address bar has http://www.google.com in it. Just replace all of that with the ip address of your router. By default the ip address should be set to 192.168.1.254.
You should see a box prompting you for your username and password. Enter your username and password now. By default both the username and password are admin. Click the Ok button to log in to your router. Please visit our default username and passsword page, if the username and password shown above did not work for you.
Click the Advanced link near the left of the page.
Click the Continue to Advanced link near the center of the page.
Click the click here link near the center of the page.
We will list a series of lines here that will show you exactly how to forward the ports you need to forward. Medieval - Total War requires you to forward the 18321 ports. Go ahead and enter the settings shown above into the Game or Application Definition menu and then click Apply.
Click the Apply button near the top of the page.
And that is it! You are done!